1. Field of the Invention
This invention relates to turbine engines and particularly to fan engines having high blade tip speeds.
2. Description of the Prior Art
The turbofan engine is the type of power plant most widely used on large aircraft today. In a conventional turbofan engine as distinguished from a turbojet engine, a portion of the working medium gases is pumped axially through one or more compression stages and is exhausted to the atmosphere without passing through the core engine. The compression stages which exhaust to the atmosphere are called fan stages and are generally positioned at the forward end of the engine. The ratio of the air flowing through the fan stages alone to the air flowing through the core engine is referred to as the bypass ratio. The bypass ratio may be a different value for each individual engine model according to the performance requirements of that power plant. In all turbofan engines, however, the fan stages make a substantial contribution to the total engine thrust at takeoff.
For large thrust contributions a bypass ratio of 5 or greater is typical. At these bypass ratios both the diameter of the fan passage and the speed of rotation of the fan are increased from conventional, lower bypass ratio engine values to pump the working medium at a sufficient flow rate. As the diameter of the fan and the speed of fan rotation are increased, the tip speed of each individual fan blade is correspondingly increased. The formula below expresses this relationship.
V.sub.t = wr PA1 V.sub.t is the velocity of the tip; PA1 w is the angular velocity of the tip; and PA1 r is the tip radius.
Where
Wherever the blade speed causes the relative velocity of the approaching medium to be in excess of Mach 1.0, the dynamic energy of the supersonically approaching medium becomes converted to pressure energy by either gradual diffusion of the medium or by shock wave phenomenon. The term "subsonic and low supersonic" as applied throughout this specification to flows generally having relative approach velocities on the order of Mach 1.5 or less and the term "high supersonic" is applied to flows generally having relative approach velocities which are in excess of Mach 1.5.
In typical turbofan engines in commercial service today, the inward portion of each fan blade sees subsonic approach flow while the outward portion of each blade sees supersonic approach flow. In the outer portion of the blade a compression shock wave, commonly referred to as an "external wave," is established upstream of each blade leading edge. Across the external wave the velocity of the approaching medium is shocked from supersonic to subsonic regimes at a substantial energy loss. The greater the velocity differential across the external shock wave in an engine, the greater the loss becomes. At relative approach Mach numbers of 2 or greater, the shock wave loss is quite substantial and severe efficiency penalties are imposed against the operating engine.
A portion of the dynamic energy is converted across a shock wave to pressure energy and is recoverable in constructions capable of containing the shock wave within the rotating blades. A contained wave is most commonly referred to as an "internal wave" and has been previously proposed, by example, in U.S. Pat. No. 2,623,688 to Davidson entitled "Rotary Power Conversion Machine." Substantial aerodynamic flow losses occur in Davidson, however, as the working medium is forced through the shock wave region.
The designers of gas turbine engines are continually searching for apparatus which will reduce the severe shock wave energy losses commonly attendant in engines with high tip speed fans.